The invention relates to the field of telemetry systems for transmitting information from the bottom of a well hole to the surface. In particular, this invention relates to the field of mud pulse telemetry where information detected down the well is transmitted to the surface by pressure pulses created in the circulating drilling fluid stream or mud stream in the drill string.
The desirability and effectiveness of well logging systems where information is sensed in the well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Systems of this type, i.e., mud pulse telemetry system, provide the driller at the surface with means for quickly determining various kinds of information down the well, most particularly information about the location and direction of the drill string at the bottom of the well.
Because of the tremendous investment already made in drill pipe and drill collars, it is highly desirable that the borehole telemetry system be compatible with existing drilling equipment and require minimum or no modification to the drill pipe and drill collars. Mud pulse telemetry is known to offer an effective solution since it does not rely upon electric wires extending to the surface, or other mechanisms which may necessitate modification to existing hardware. Mud pulse telemetry propagates signals through the mud flow in the drill string to the surface at the speed of sound, thereby providing a very fast communication link between the drill bit and the surface. Mud pulse telemetry is usually in the form of a valve located in the vicinity of the drilling bit, which intermittently restricts the flow of mud within the drill string. The telemetry system may be lowered on a wireline located within the drill string, but is usually formed as an integral part of a special drill collar inserted into the drill string near the drilling bit.
A continuous column of mud is circulating within the drill string from the surface of the well to the drill bit at the bottom of the wall during normal drilling operations. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through the downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well at the speed of sound through the drilling mud. The information sensed in the vicinity of the drilling bit, which is to be transmitted to the surface, is encoded into a digital format and that digital formatted signal is used to intermittently actuate the telemetry valve which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface. The pulses are detected at the surface and transformed into electrical or other signals which can be decoded and processed to reveal transmitted information. In a typical oil or gas well drilling mud is circulated through the interior of the drill pipe at flow rates of about 100 to 1200 gallons per minute. The mud pulse telemetry system must operate to partially restrict this flow, therefore the system must control large amounts of energy. The telemetry valve must operate quickly to create a pressure pulse in this high pressure environment to intermittently restrict the flow of mud. This restriction must be sufficient to create a pressure rise in the flow stream that will be detectable at the surface of the well. At the typically high flow rates of mud, considerable force and energy are required to actuate the telemetry valve in the manner necessary to create the desired pressure pulses.
A telemetry system which is capable of performing the desired function with a small amount of control energy is extremely desirable. Such a system should lend itself to size reduction and/or miniaturization that can be easily packaged within the confines of conventional drill pipe segment or drill collar. Furthermore, if input power requirements are low enough, downhole power sources such as high temperature batteries can be used to power the telemetry system.